The present invention relates to a magnetic material having a high magnetic permeability in a high-frequency region and, in particular, to an electromagnetic noise suppressing thin film which suppresses or absorbs a high-frequency current acting as noise.
Following rapid spread of a mobile telephone and development of a wireless LAN, an electromagnetic environment problem including an influence of an electromagnetic wave upon a human body becomes publicly recognized. Under the circumstances, attention is increasingly directed to “electromagnetic compatibility, EMC” considering both suppression of emission of the electromagnetic wave from a wide range of electronic devices within a certain limit and improvement of noise immunity against an influence from the surroundings.
In recent years, the EMC is directed to electronic apparatuses, such as a mobile telephone or a CPU used in a PC, operable in a quasi microwave band (several hundreds MHz to several GHz). For the EMC in such a high-frequency region, it is an urgent need to develop a material having a high magnetic loss (μ″) and a magnetic resonance frequency in the above-mentioned high-frequency region as well as a high electric resistance.
As typical conventional magnetic materials that can be used at a high frequency, there are known a ferrite, a metal thin film, a multilayer film comprising a combination of a metal and a nonmagnetic insulating material, a granular thin film, and the like.
The ferrite has a very high electric resistance and occurrence of an eddy current at a high frequency is very low. Therefore, the ferrite can be used in bulk. However, at a high frequency of several tens MHz or more, resonant vibration of a domain wall and a spin resonance phenomenon occur so that a so-called Snoek's limit appears. In order to further increase the frequency, it is effective to form a thin film of several μm or less so that a shape magnetic anisotropy is increased to thereby raise the Snoek's limit. However, formation of a ferrite phase having a high magnetic permeability requires a process at about 1000° C. In this event, the thin film is difficult to form. Therefore, practical application has never been reported.
The metal thin film typically uses a permalloy (Ni80Fe20) or an amorphous metal and achieves a very high magnetic permeability. However, because of its very low electric resistance, an eddy current tends to be generated. Therefore, at a higher frequency, the thickness of the metal thin film is limited to be smaller. Particularly at the frequency on the order of GHz or more, the problem of the eddy current occurs unless the thickness is 0.1 μm or less.
In view of the above, use is made of the above-mentioned multilayer film, i.e. a thin film material in which metal thin films and insulator thin films such as oxide thin films are laminated so as to suppress occurrence of the eddy current. However, since the magnitude of overall magnetization is reduced and a fabrication process becomes complicated, use of the multilayer film is limitative.
The granular thin film is a recently-developed thin film having a granular structure. In the granular thin film, fine particles of a ferromagnetic metal having an isotropic shape approximate to a spherical shape are dispersed in a matrix such as oxide, thereby realizing an electric resistance (up to 103 [μΩcm]) higher than that of a metal by several figures. The granular structure is a structure in which fine magnetic metal particles having a diameter of 10 nm or less are deposited in the oxide. Specifically, the granular structure is produced by a thin film fabrication technique such as sputtering. The granular thin film can have a high electric resistance and a strong magnetic anisotropy due to anisotropic coupling of the fine particles. It is therefore possible to suppress or control occurrence of a spin resonance phenomenon in a GHz band. Accordingly, the granular thin film is considered to have a wider application range as compared with the conventional thin film materials.
However, it has been found out that, when the granular thin film is disposed near a transmission line for use as an EMC measure, reflection of a signal transmitted through the transmission line becomes remarkable in the high-frequency band of several hundreds MHz or more. In order to prevent such reflection, the electric resistance must be yet higher by another several figures.
Thus, the granular thin film must have a yet higher electric resistance. However, there are following problems.
First, each of the fine ferromagnetic metal particles forming the granular structure itself has a diameter of several nanometers and, in an isolated state, loses ferromagnetic properties due to thermal agitation at room temperature (a phenomenon called superparamagnetism). In order to make the fine particles have the ferromagnetic properties, magnetic coupling is induced between the fine particles to thereby overcome the thermal agitation. In this event, the fine particles show magnetic properties as a group behavior due to the magnetic coupling and exhibit a high magnetic permeability. Thus, in order to obtain the properties as a high-magnetic-permeability thin film, the magnetic coupling between the fine particles is essential and indispensable. For the magnetic coupling, the presence of metallic coupling between the fine particles in the insulating material is necessary. Such metallic coupling causes a decrease in electric resistance. Thus, the high magnetic permeability and the high electric resistance are conflicting parameters. Accordingly, in the granular structure, the electric resistance has an upper limit.